Pulsed neutron logging system

ABSTRACT

An illustrative embodiment of the invention includes improved nuclear well logging methods and apparatus for investigating subsurface earth formations. A pulsed neutron generator is used to irradiate the earth formations and the gamma rays caused by the inelastically scattered neutrons are observed in four energy windows. The carbon/oxygen ratio and the silicon/calcium ratio and the counting rate in each window are obtained and used to resolve ambiguities in the oil/water content of the formations by appropriately combining the measurements to obtain the lime factor or fraction and the water saturation SW of the formations, which quantities are then recorded as a function of the borehole depth.

United States Patent n 1 Arnold et al.

[ PULSED NEUTRON LOGGING SYSTEM [75] Inventors: Dan M. Arnold; Ward E.Schultz;

Harry D. Smith, Jr., all of Houston, Tex.

[73] Assignee: Texaco Inc., Houston, Tex.

[22] Filed: Sept. 20, 1971 [2| I App]. No.: l82,035

[52] U.S. Cl. .f. ..250/30l, 250/270 [5|] lnt.'Cl. G01t 1/16 [58] Fieldof Search 250/836 W, 83.6 S, 250/833 R, 71.5 R

[56] References Cited UNITED STATES PATENTS 3,483,376 l2/l969 Locke etal. 25()/83.6 W X [111 3,780,302 [4 1 Dec. 18, 1973 PrimaryExaminer-Walter Stolwein Assistant Examiner--Davis L. WillisAttorney-Thomas H. Whaley et al.

[57] ABSTRACT An illustrative embodiment of the invention includesimproved nuclear well logging methods and apparatus for investigatingsubsurface earth formations. A pulsed neutron generator is used toirradiate the earth formations and the gamma rays caused by theinelastically scattered neutrons are observed in four energy windows.The carbon/oxygen ratio and the silicon/calcium ratio and the countingrate in each window are obtained and used to resolve ambiguities in theoil/water content of the formations by appropriately combining themeasurements to obtain the lime factor or fraction and the watersaturation S of the formations, which quantities are then recorded as afunction of the borehole depth.

19 Claims, 4 Drawing Figures 5w Q i /o i i Si/ 2 9 i .v 41 7 WATER 30SATURATION SW SW COMPUTER 43 cul /o i i a l. a. VA 7.- 4m. .17. LPOROS/TY LIME FRACIION g COMPUTER /LF)COMPUTER it 7 L L L L ELLEN" L V Vl i TIME C w 3 -C/o RAT/0 J PULSE C0MPUTER GATE HEIGHT s L! 3 ANALYZER.59, i e. ECTRJM i M 4 STABlL/ZER 23 I PATENIEDBEB 18 ms v 4 sum 2 or 2GAMMA RAY ENERGY (MEV) PULSED NEUTRON LOGGING SYSTEM BACKGROUND OF THEINVENTION This invention relates to radiological well logging methodsand apparatus for investigating the characteristics of subsurface earthformations traversed by a borehole and, more particularly, relates toimproved neutron-gamma ray logging methods and apparatus.

It is well known that oil and gas are more likely to be found incommercially recoverable quantities from those earth formations whichare relatively porous and permeable than in more highly consolidatedearth formations. It is also well known that an oil or gas producingformation may be located by passing a neutron source through theborehole and measuring the intensity of secondary gamma ray radiationdeveloping from the neutron irradiation as a function of borehole depth.

In particular a chlorine nucleus, which has a very high thermal neutroncapture cross sectiontmore so than that of the nuclei of other rathercommonly found elements) is a good indicator of the location of saltwater. Thus, salt water filled limestone or sandstone layers will have agreater macroscopic thermal neutron capture cross section than an oilsaturated layer will. When combined with otherporosity information, oilcan thus be detected. This difference has been observed in the past bymeasuring either chlorine capture gamma rays or the lifetime or decayconstant of the thermal neutron population in the layer in question.

The above-mentioned salt water detection techniques have proven to bevery useful in the past in locating oil and gas bearing earthformations. However, many spurious indications have been produced bythis logging technique due to the fact that it depends on the presenceof a rather large amount of sodium chloride in the fluid. There has beenno commercially available well logging method which could distinguishoil from water in earth formations when the water salinity is low. Forexample, the above-mentioned chlorine or neutron lifetime logs requirewater salinities in excess of about 30,000 parts per million of sodiumchloride before oil located in the pores of the formation can bedifferentiated from water.

Accordingly, it has been proposed in the prior art to make a measurementof at least a portion of the gamma ray energy spectrum due to inelasticneutron scattering events from neutron irradiated earth formations. Thishas been proposed because carbon and oxygen have significant inelasticcross sections while having relatively small capture cross sections.Thus, the carbon and oxygen nuclei in the earth formations surroundingthe borehole will engage in appreciable inelastic scat teringinteractions with the bombarding neutrons. Gamma rays resulting frominelastic neutron scattering interaction will be referred to henceforthas inelastic gamma rays. However, this approach has been limited in thepast to some extent because the inelastic scattering cross section forcarbon and oxygen only become appreciable if relatively high energyneutrons are available to provide the interaction. In the past it hasbeen difficult to provide sufficient quantities of energetic neutrons toreliably perform this type of log. The development of improved pulsedneutron generators has made possible the measurement of the inelasticscattering gamma ray energy spectrum from relatively high energy neutronirradiated earth formations. Attempts have been made to measure thecarbon and oxygen inelastic scattering interactions with l4 MEV neutronsgenerated in pulsed neutron generators of the deuterium-tritium reactiontype.

To the present, however, none of the proposed methods utilizing thisconcept have proven reliable. One of the main reasons for lack ofsuccess in these attempts has been that carbon is present in significantamounts in the earths crust. Moreover, limestone formations are largelycomposed of calcium carbonate and thus a water bearing limestoneformation can produce more carbon gamma rays due to inelasticscatterings than an oil filled sand or shale. The carbon/oxygen ratiohas alsobeen found to be a function of porosity. However, it has alsobeen found that the water saturation of earth formations may be found inthe manner to be described, by measuring the carbom'oxygen ratio.

Another problem in making inelastic gamma ray measurements has been dueto the fact that the gamma ray generated by the neutron inelasticscatteringcan itself engage in multiple Compton scattering interactions.Such gamma ray scattering generally tends to make the scattered gammaray lose energy to some extent with each interaction. Thus, a gamma rayhaving a particular initial energy generated by the inelastic scatteringof a neutron by a carbon or oxygen nucleus can have a totally differentenergy (which could be assumed to be its initial energy) by the time itreaches the detector in the logging sonde. This. type of processgenerally masks or smears inelastic gamma ray energy spectra.

Even if the improved pulsed neutron sources which are now available areused to perform the inelastic neutron scattering log, the neutron outputmust be limited to a relatively small number of neutrons during eachpulse so that pulse pile up in the system electronics will not destroythe signal and resolution from formation gamma rays. Pulse pile upresults from the fact that the system, including the electronic circuitsand the well logging cable, have only the capability to effectivelycount at a finite instantaneous counting rate. Also, in this regard, theneutron source to gamma ray detector spacing becomes particularlycritical in regulating the count rate so as not to exceed the finiteinstantaneous count rate limitation of the system. Thus, in order toenhance the statistical accuracy of the measurements (which generally isa function of the total number of counts) the source/detector spacingand pulse repetition rate of the pulsed neutron source may be varied toobtain the optimum results; the duration of each individual neutronpulse is also made as short as possible (5 microseconds). It is possibleto obtain a source/detector spacing and pulse repetition rate which willpro vide a maximum number of total counts while not exceeding theinstantaneous count rate limitation of the system during the individualinelastic gamma ray measuring intervals which coincide with the neutronpulse duration. Also, keeping the pulse duration as short as possiblecuts down the count rate due to thermal neutron capture events since, aswill be discussed subse-.

quently, there is not a long enough time duration for such thermalneutron background to build up during the pulse. However, increasing thepulse repetition rate too much can lead to the existence, during thecounting interval for inelastic gamma rays, of a large thermal neutronpopulation from a previous neutron pulse. This problem can be dealtwith, however.

It will be appreciated by those skilled in the art, that the foregoingdiscussion of the parameters which may be varied to optimize the loggingspeed and counting statistics are interdependent, not only with eachother but also with physical parameters of the well bore being examined.For example, the bore hole diameter, relative thickness of casing andcement sheath and type of borehole fluid can all effect the inelasticgamma ray count rates. Thus, in order to obtain reasonable count ratesat reasonable source/detector spacing and still maintain resolvablegamma ray pulses which are not seriously deteriorated by the pulsepileup phenomena, it is desirable to repeat the neutron pulses at a highrate.

Accordingly, it is an object of the present invention to provide animproved method and apparatus for obtaining information indicating thepresence of oil bearing formations in relatively low salinity earthstructures.

A further object of the invention is to provide new and improved methodsand apparatus for determining the carbon/oxygen ratio in earthformations surrounding a well borehole.

A still further object of the invention is to provide a well loggingsystem which is a direct indicator of oil in earth formationssurrounding a well borehole independently of the presence of chlorine inthese formations.

Yet another object of the present invention is to provide an improvedwell logging system indicative of the presence of oil and formationlithology and porosity and water saturation.

The above and other objects, features and advantages of the presentinvention are provided in a pulsed neutron well logging system. Thesystem utilizes four energy dependent windows or intervals in the gammaray energy spectrum. Time dependent gate means isolate gamma raysresulting from inelastic scattering of neutrons by earth formationssurrounding the well borehole. Four energy windows in the inelasticgamma ray energy spectrum are positioned and their width chosen so thatinelastic gamma rays from carbon, oxygen, silicon and calcium aredetected. Gamma rays occuring in the carbon and oxygen windows or anycombination of the windows may be utilized as an indicator of formationporosity. The carbon/oxygen ratio detected by the present invention canbe indicative of the presence of hydrocarbons in the pore space of theearth formations and the silicon/calcium ratio can be indicative of theformation lithology. By appropriately combining measurements of thecarbon/oxygen ratio and the silicon/calcium ratio and estimates of theformation porosity, the water saturation S of the formation matrixsurrounding the borehole may be computed and logged as a function ofborehole depth of the well tool. Optimal source to detector spacings,neutron pulse time duration, and neutron pulse repetition rates forachieving the above results are disclosed.

The above and other objects, features and advantages of the presentinvention are pointed out with particularity in the appended claims. Thepresent invention is best understood by taking the following detaileddescription in conjunction with the appended drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an overall schematic blockdiagram of a well logging system in accordance with the invention.

FIG. 2 is a timing diagram showing the relationship of accelerator anddetector on time with respect to gamma rays caused by the inelasticscattered neutrons and the thermal neutrons in the vicinity of theborehole.

FIG. 3 shows a graphical representation of a gamma ray spectrumresulting from the inelastic scattering of neutrons and showing therelative location of the Si, Ca, C, and O inelastic gamma ray energywindows.

FIG. 4 is a graphical representation showing the C/O ratio as a functionof formation porosity for several lithologies and water saturations, asdetermined from test formation measurements.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring initially to FIG. 1there may be seen a simplified functional representation in the form ofa block diagram of well logging apparatus in accordance with the presentinvention. A borehole 2 penetrating earth formations 3 is lined with asteel casing 4 and fllled with a well fluid 5. The steel casing 4 iscemented in place by a cement layer 6 which also serves to prevent fluidcommunication between adjacent producing formations in the earth 3.

The downhole portion of the logging system may be seen to be basicallycomposed of an elongated, fluid tight, hollow body member or sonde 7which, during the logging operation is passed longitudinally through thecasing 4 and is sized for passage therethrough. Surface instrumentationwhose function will be discussed in more detail subsequently is shownfor processing and recording electrical measurements provided by thesonde 7. A well logging cable 8 which passes over a sheave wheel 9supports the sonde 7 in the borehole and also provides a communicationpath for electrical signals to and from the surface equipment and thesonde 7. The cable 8 may be of a conventional armoured well logging typeand may have one or more electrical conductors for transmitting suchsignals between the sonde 7 and the surface apparatus.

Again referring to FIG. 1, the sonde 7 contains a source of high energyneutrons 11. The neutron source contemplated for use herein is a pulsedneutron source operating from the principle of the deuterium-tritiumreaction. However, it will be understood by those skilled in the artthat the invention is not limited thereto. Other types of pulsed neutronsources may be used if desired. A suitable radiation detector comprisinga photomultiplier tube 10 and a detector crystal 12 is provided in thesonde 7 for detecting gamma rays resulting from the inelastic scatteringof high energy neutrons by the earth formations 3 surrounding the wellborehole 2. A radiation shield 13 of iron, lead or other suitablematerial is interposed between the neutron accelerator 11 and thedetector crystal 12 of the apparatus. Additionally a thermal neutronshielding sleeve 15 may be provided as shown about the detector crystal12 on either the interior or the exterior wall portion of the sonde. Athermal neutron shielding disc 16 is interposed between the radiationshielding material 13 and the detector crystal 12 to reduce theprobability of thermal neutrons reaching the detector crystal. Thedetector crystal 12 may comprise a thallium doped sodium iodide, cesiumiodide or other like activated material which is optically coupled tothe photomultiplier 10.

The radiation shielding 13 reduces the probability of direct irradiationof the detector crystal by neutrons emitted from the pulsed neutronsource or accelerator l I. The thermal neutron shielding disc 16 andcylinder surrounding the detector crystal may be comprised of boron orany other suitable material having a high thermal neutron capture crosssection. This shield serves to further reduce the possibility of thermalneutrons which follow a tortuous path and have been slowed by theborehole fluid 5 or the shielding material 13 from reaching the vicinityof the detector crystal and possibly'causing neutron activation of theiodine or other elements comprising the crystal. Moreover, the thermalneutron shield reduces the probability of thermal neutrons from aprevious accelerator neutron pulse interacting with materials in thesonde itself or the detector crystal itself and causing the emission ofgamma radiation during thetime period when the inelastic neutron gammarays are being observed.

As is well known in the art, the scintillation crystal 12 produces adiscrete flash of light whenever a gamma ray passes therethrough andexchanges energy with its crystal lattice. The photomultiplier tube 10generates a voltage pulse proportional in height to the intensity ofeach such scintillation which occurs in the crystal 12. The intensity ofsuch scintillations is functionally related to the energy of the gammaray causing the light flash and thus a voltage pulse generated by thephotomultiplier tube 10 has an amplitude functionally related to theenergy of the corresponding gamma ray. These proportional voltage pulsesproduced by photomultiplier tube 10 comprise a detector signal which issupplied to a linear amplifier 17 via a discriminator 18. Thediscriminator 18 may be used if desired to discrim inate, for example,against low energy background gamma radiation resulting from the thermalactivation of the detector crystal by the reaction I? (N I A preset biaslevel may be used to pass only pulses from the photomultiplier tube 10exceeding the height corre sponding to 1.78 MEV gamma rays generated inthe inelastic scattering of neutrons by silicon. Low energy backgroundgammas contributing to pulse pileup could be eliminated in this manner.Also, the discriminator, being located downhole, reduces the counts ratesupplied to the cable, thus enhancing the possibility of not havingpulse pileup.

The neutron accelerator l 1 is preferably operated by a pulsing circuit14, which may be of conventional design as known in the art, andfunctions to operate the acclerator in short duration pulses. Thepulsing circuit 14 may be controlled by timing pulses from a surfacetiming reference 39 communicated over the cable 8 conductors and whichmay also be supplied to a downhole reference pulser 20. For example, thepulsing circuit 14 could be activated by a timing pulse from timereference 39 to emit a neutron burst of a specified time duration. Thefrequency of such bursts would then be controlled by the surface timingreference 39. Timing reference 39 may also be located in the sonde, ifdesired. In the inelastic scattering measurements contemplated in thepresent invention it would be desirable to use neutron pulses of aboutfive microseconds duration and which are repeated at periodic intervalsfrom approximately 5,000 to 20,000 or more times per second. I Referringnow to FIG. 2 the relative time relationship of the neutron bursts orpulses just discussed to the operative gamma ray detector cycle and tothe inelastic andthermal neutron populations are shown. The acceleratorcycle is represented by the: solid curve 33. The inelastic gamma raypopulation in the vicinity of the detector crystal 12 is represented bythe dashed curve 31. The thermal capture gamma ray population in thevicinity of the detector is shown by the dotted line curve 32. Theoperative detector cycle is represented by the separate solid line curve34. It will be noted that when the five microsecond neutron pulse ofcurve 33 commences that the detector has already been activated forapproximately one microsecond as indicated by curve 34. This isaccomplished by supplying the timing from time reference 39 to a signalgate 22 prior to supplying it, via the cable 8, to the downhole pulser14. i

There is a sudden and rapid build up of the inelastic gamma raypopulation (curve 31) which is essentially present only during theneutron pulse. The thermal capture gamma ray population (curve 32)builds up much slower and reaches a peak only after the cessation of the5 microsecond neutron pulse. In the diagram of FIG. 2 it will be notedthat the neutron pulses are spaced sufficiently far apart in time forthe thermal neutron population (curve 32) to decay to almost zero beforethe next pulse. However, in general some neutrons are still extant inthe vicinity of the detector at the time the subsequent neutron pulseoccurs. In this case the boron thermal neutron shielding material 15 anddisc 16 is particularly adapted for reducing spurious gamma raydetections resulting from this source by rapidly absorbing such thermalneutrons.

Referring again to FIG. 1 and bearing in mind this timing sequence, itwill be observed that during the time the neutron accelerator 11 isactivated, output signals from the photomultiplier tube 10 are conductedvia a discriminator 18 and a linear amplifier 17 to a cable drivercircuit 19 of conventional design. A reference signal having a knownamplitude is also supplied by a pulser 20 to the input of thediscriminator 18. The reference pulse provided by the downhole pulser 20is utilized in a gain control device or spectrum stablizer 23 to controlthe gain of the system in the manner described in the copending patentapplication Ser. No. 82,028 filed Oct. 19, 1970. This, of course, may beaccomplished primarily between neutron pulses as the spectrum stabilizer23 may be supplied with signals from the pulser '20 continuously or inany desired sequence.

Since both the downhole pulse generator 14 and the surface gate 22 whichcontrol data pulses from the downhole tool are timed from the same timereference 39, it is apparent that synchronism may be maintained betweenthe surface equipment and the downhole equipment. Thus the data signalsmay be gated in a manner at the surface to select portions thereof forprocessing which are timed as desired relative to the emission of theneutrons in the manner previously discussed with respect to FIG. 2.

Although not depicted in FIG. 1, it will be understood by those skilledin the art that electrical power may be supplied from a surface powersource (not shown) via the well logging cable 8 to the downhole sonde 7.Suitable power supplies (not shown) are provided in the sonde 7 forpowering the downhole portion of the equipment.

The output signals from the gate 22 comprise a sequence of count pulsesresulting from gamma rays de' tected by the downhole detector crystal 12during the time interval that the neutron accelerator 11 is activated.These pulses comprise data mainly from gamma rays resulting from theexitation of nuclei in the vicinity of the detector crystal which havebeen excited by the inelastic scattering of neutrons emitted byaccelerator 11.

The inelastic gamma rays are supplied to a pulse height analyzer 24. Thepulse height analyzer 24 may be of conventional design as known in theart and having, for example, four or more channels or energy divisionscorresponding to quantizations of the pulse heights of the input pulses,if desired. The pulse height analyzer 24 functions to sort andaccumulate a running total of the incoming pulses into a plurality ofstorage locations or channels based on the height of the incoming pulseswhich, it will be recalled, is directly related to the energy of thegamma rays causing the pulse. The output of the pulse height analyzer 24in the case of the present invention consists of count pulses occurringin each of four energy ranges or windows as depicted in FIG. 3.

Referring now to FIG. 3,'the relative count rate as a function of energyis shown directly as curve 51. Curve 52 indicates the same relativecount rate multiplied by a factor of 4 so that details may be moreaccurately seen. It will be observed that four energy windowscorresponding to silicon (labeled Si), calcium (labeled Ca), carbon(labeled C) and oxygen (labeled are provided. In the practice of themethod of the present invention, it has been found preferable to use acarbon energy window extending from 3.17 to 4.65 MEV. The oxygen energywindow preferably extends from 4.86 to 6.34 MEV. The silicon window ischosen to extend from 1.65 to 1.86 MEV and the calcium window extendsfrom 2.5 to 3.3 MEV. By using this choice of energy windows the 4.4 MEVcarbon inelastic gamma ray scattering peak and the 6.13 MEV oxygeninelastic gamma ray energy scattering peaks are covered, togehter withtheir corresponding single and double pair production escape peaks. Theoptimal choice of energy range for the silicon window encompasses the1.78 MEV photo peak for silicon. The calcium energy window illustratedin FIG. 3 does not encompass the 3.73 MEV calcium photo peak due tocarbon interference, but does encompass the corresponding single anddouble escape peaks. It will be understood, of course, that slightvariations of this placement of energy windows could be made if desiredwithout compromising the inventive concepts drastically.

The number of counts occuring in each of the four energy windows duringthe time interval (7 microseconds) that the downhole detector signalsare supplied to the pulse height analyzer via the gate 22 are outputfrom the pulse height analyzer 24 as four separate digital signals. (Itwill be understood, of course, that the individual time interval pulsecounts may be integrated over a longer period for better statisticalaccuracy as known in the art.) The carbon and oxygen count rates aresupplied to the carbon/oxygen ratio computer 25. The silicon and calciumcount rates are supplied to the silicon/calcium ratio computer 26.Additionally the count rates in the C, O, Si, and Ca windows are fed toa porosity computer 27 whose function will be subsequently described.The ratio computers 25 and 26 may be of conventional design as known inthe art.

The silicon/calcium ratio from the ratio computer 26 is supplied to alime fraction computer 28. The lime fraction computer 28 output togetherwith the output of the porosity computer 27 and the CIO computer 25output is supplied to the water saturation computer 29. The functioningof the computers 27, 28 and 29 will be described in more detailsubsequently. The porosity computer 27 estimates the porosity as afunction of the count rate in an individual energy window such as carbonor in a combination of energy windows such as C+Ca, or all four, ifdesired, and supplies an output signal proportional thereto. This outputsignal may be recorded together with the silicon/calcium ratio, thecarbon/oxygen ratio and a water saturation S signal generated by thecomputer 29. These signals can be recorded as a function of boreholedepth by the recorder 30 which is mechanically or electronically linkedto the sheave wheel 9 as indicated by the dotted line 42. A log 41 ofthese quantities as a function of borehole depth is illustratedschematically.

It may be shown that the depth of investigation and the relative amountof formation signal increase gradually with an increased neutron sourceto detector spacing. However, the number of inelastic gamma raysreaching the detector decreases rapidly with increased spacing. For thedetector and deuterium-tritium accelerator used at 5,000 bursts persecond, it has been determined that at spacings less than 24 inchesthere is a considerable amount of pulse pile up (too rapid counting)during the period that the inelastic gamma ray gate 22 is open. Aspreviously discussed, this can lead to a loss of resolution in theenergy spectrum of the gamma rays being measured. At source to detectorspacings greater than 24 inches at 5,000 pulses per second the number ofinelastic gamma rays reaching the detector has been found to diminish inan unfavorable manner.

For this reason it has been found desirable to use a spacing ofapproximately 24 inches from the source to the detector in order toachieve optimum counting results with the instrument at 5,000 pulses persecond. Of course, this distance would be changed if improved generatorsor detectors are made available, or if higher repetition rates for theneutron generator, with a smaller number of neutrons per burst, areemployed. For example, if the pulse repetition rate is increased to20,000 pulses per second, then a spacing of approximately 22 inches hasbeen found satisfactory while avoiding pulse pile up problems.

Measurements made under test conditions using a well logging system suchas that illustrated in FIG. 1 over a plurality of different types oftest formations have been found to yield results illustrated graphicallyin FIG. 4. In FIG. 4 the curve labeled f represents the relative ratioof counts in the carbon and oxygen count windows for a plurality of adifferent sandstone forma tion porosities at percent water saturation.The curve labeled f represents the carbon/oxygen counts ratio insandstone formations at 0 percent water saturation. Similarly, thecurves f and fl represent measurements obtained in limestone formationsof varying porosities with 100 percent and 0 percent water saturation,respectively. The dotted line curves 54 and 53 represent the 50 percentwater saturation carbon/oxygen ratio logs for limestone and sandstoneformations, respectively. From the graphs of FIG. 4 it is apparent thatin high porosity formations a carbon/oxygen ratio log by itself would beanomalous as an oil indicator, since, for example, at porosities greaterthan 30 percent, a carbon/oxygen counts ratio in the range of 1.6

to l.75 could be either a water saturated limestone as represented bycurve f or an oil saturated sandstone as represented by curve f I Inorder to resolve such anomalous results the silicon/calcium ratio may beused in order to determine the approximate lithology of the earthformations. It is apparent that the silicon/calcium ratio will varydramatically when going from a sandstone (SiO to a limestone (CaCOformation. In going from a sandstone to a limestone formation thecalcium content increases while the silicon content decreases. Thisleads to a much lower silicon/calcium ratio. The inverse is true, ofcourse, in going from a limestone to a sand stone formation. Thus, theSi/Ca ratio curve within a formation is indicative of lithology.Additionally, in order to determine accurately whether water or oilexists in the pore space of the formations some indication of theporosity of the formations is necessary.

The first step in determining water saturation S is to quantitativelydetermine the fractional concentration of sand and lime in the formationmatrix. To do this, it is sufficient to determine the relative amountsof silicon and calcium present. Taking the ratio of the count rates inthe energy windows previously described for Si and Ca has been found toeffectively indicate the matrix and, at the same time, is virtuallyinsensitive to porosity, water saturation and water salinity. If theSi/Ca values are determined for a pure water sand in the well (R and apure water lime (R, then these two values may be used as calibrationpoints and the fraction of limestone in an unknown sandy lime whoseSi/Ca ratio (Rx) is measured can be established from the relationship(LIME FRACTION)X LFX (R RAJ/(RS RL) (1) This computation may beperformed in the lime frac tion computer 28 of FIG. 1, using themeasured Si/Ca ratio as indicated in FIG. 1 for input. Representativevalues for R and R may be determined by measuring the Si/Ca ratio (R ina known water sand in the well at the beginning of the logging run. R isknown to generally be about percent greater than R and thus once R,- isknown for the well, R may be obtained. When R is measured then all ofthe right side of Equation (I) is known and LF may be computed.

The second step in determining the water saturation S is to obtain aporosity estimate 5 of the formation. Porosity may be estimated from thecount rates observed in any (or all) of the previously defined energywindows from the expression where M and N are constants determined byborehole parameters and neutron output. Of course, e=2.7l8-

water saturation so, if desired.

From the graph of FIG. 4 it may be seen that if the curvesf f ,f andf,are thought of as being functions of porosity 4), then to a 'goodapproximation where the constant K is a measure of the increase in thecarbon/oxygen ratio caused by the carbon in the limestone matrix and isthus indicative of the amount of limestone present. Thus thecarbon/oxygen ratio in a formation with a given water saturation Scomposed of Y% lime and Y)% sand would differ from the carbon/oxygenratio in a 100 percent sand with the same water saturation and porosityby a factor (K Y/ 100).

From Equation (1), LF, is also an indicator of the percentage limestonein the unknown formation X and thus LE Y/IOO. If(K LF is subtracted fromthe carbon/oxygen ratio observed in an unknown formation X the resultingcarbon/oxygen ratio value is representativeof a pure sand having thesame porosity 5 and water saturation S as formation X. The watersaturation S may then be derived by linear interpolation between curvesfandf of FIG. 4.

The water saturation computer 29, using as inputs LF and (I) as derivedabove from Equations l and (2) then perform the computation (K LF,) andthe linear interpolation between f, and f For this purpose f and f maybe approximated as polynominals in 12 so that the computer may evaluatethese functions analytically. The resultant water saturation S may thenbe thought of as the pure sand equivalent water saturation.

Alternatively, if desired, (not shown) the outputs of the pulse heightanalyzer 24, FIG. 1 could be recorded in digital or analog form at thewell and later input to a general purpose digital computer directly andthe indicated computations performed. The resultant outputs from thegeneral purpose computer could then be used to drive the recorder orother display means. In either case the water saturation S porosity d),C/O ratio, and Si/Ca ratio may be logged (41) as a function of boreholedepth. Thus, the count rates in the four energy windows of FIG. 3 may beutilized in the technique of the present invention to resolve:ambiguities which would heretofore have been unresolvable by combiningthese data in the manner disclosed.

The above disclosure may make other alternative embodiments of theinvention apparent to those skilled in the art. It is the aim of theappended claims to cover all such changes and modifications as fallwithin the true spirit and scope of the invention.

We claim:

1. A method of determining the water saturation of earth formationssurrounding a well bore comprising the steps of:

determining the fractional content of sand and lime in the formationmatrix;

determining the porosity of the formation;

determining the carbon/oxygen counts ratio in the inelastic gamma rayenergy spectrum in regions thereof corresponding to carbon and oxygeninelastic gamma ray and combining the fractional content of sand andlime,

the porosity of the formation, and the carbon/oxygen counts ratio in apredetermined manner to compute the water saturation of the matrix.

2. The method of claim 1 wherein the step of determining the fractionalcontent of sand and lime in the formation matrix is performed byrepetitively irradiating the matrix with high energy neutron pulses andmeasuring gamma radiation due to the inelastic scattering of neutrons bythe formation matrix in plural energy ranges.

3. The method of claim 2 wherein said plural energy ranges are chosen toinclude gamma radiation resulting from the inelastic scattering of highenergy neutrons by carbon, oxygen, silicon and calcium nuclei.

4. The method of claim 3 wherein the energy ranges are chosen to be 3.17MEV to 4.65 MEV for carbon, 4.86 MEV to 6.34 MEV for oxygen, 1.65 MEV to1.86 MEV for silicon and 2.5 MEV to 3.3 MEV for calcium.

5. The method of claim 1 wherein the step of determining the limefraction LF of an unknown formation X, is performed by measuring thecounts ratio R of silicon/calcium counts resulting from gamma radiationdue to inelastic fast neutron scattering in energy regions in the gammaray spectrum corresponding to the production of such gamma radiation bysilicon and calcium nuclei in the unknown formation, and combining thismeasurement according to the relationship where R is the silicon/calciumcounts ratio taken with the same instrumentation in a known water sandformation and R, is the silicon/calcium counts ratio taken with the sameinstrumentation in a known water lime formation.

6. The method of claim 5 and further including the step of logging thelime fraction Llas a function of borehole depth.

7. The method of claim 1 wherein the step of determining the formationporosity is done by computing the porosity q in a predetermined manneras a function of the count rates CR in energy regions of the gamma rayenergy spectrum corresponding to known gamma ray energies produced bythe inelastic scattering of high energy neutrons by the nuclei ofcarbon, oxygen, silicon and calcium.

8. The method of claim 7 wherein the porosity d) is computed from saidcount rate CR according to the relationship where M and N arepredetermined calibration constants for a particular logging tool andborehole size and the constant e 2.718- the Naperian logorithm base.

9. The method of claim 8 wherein CR is the count rate from the carbonwindow only.

10. The method of claim 8 and further including the stepof logging theporosity 5 as a function of the. borehole depth of a well tool.

11. The method of claim 1 wherein the step of combining the fractionalcontent of sand and lime, the formation porosity, and the carbon/oxygencounts ratio to compute the water saturation S of the formation isperformed by linearly interpolating by an amount (K-LF where K is apredetermined constant and LF 12 is the measured lime fraction in theunknown formation, between two preselected carbon/oxygen counts ratiofunctions, f and f where f,() is a function of porosity expressing thecarbon/oxygen counts ratio in a water sand and f is a function ofporosity d), expressing the carbon/oxygen counts ratio in an oil sand.

12. The method of claim 11 wherein the function f,() and f arepreselected to be polynomial functions of porosity 4).

13. The method of claim 11 and further including the step of logging thewater saturation S as a function of the borehole depth of a well tool.

14. The method of claim 1 and further including the step of logging thewater saturation S the porosity the carbon/oxygen counts ratio and theSi/Ca counts ratio all simultaneously as a function of the boreholedepth of a well tool.

15. A method of determining the characteristics of earth formationspenetrated by a well borehole comprising the steps of:

passing a well tool having a pulsed neutron source and a gamma raydetector through a well bore;

repetitively irradiating the earth formations sur rounding the well borewith high energy neutrons;

measuring the count rates of inelastic gamma rays in the gamma rayenergy spectrum in energy regions thereof corresponding to carbon,oxygen, silicon and calcium nuclei excited by the high energy neutronpulses;

determining the carbon/oxygen count rate ratio the formation; Idetermining, from the silicon and calcium count rates, the lime fractionof the formation; determining, from at least one of the above countrates, the formation porosity; and combining the lime fraction, thecarbon/oxygen ratio and the porosity data to derive the water saturationof the formations penetrated by the borehole.

16. The method of claim 15 wherein the neutron pulses have as short aduration as practicable to eliminate interference in the inelastic gammaray energy spectrum from the buildup of a thermal neutron population.

17. The method of claim 15 and further including the step of recordingthe water saturation S the porosity d), the CIO ratio and the Si/Caratio as a function of borehole depth of said well tool as it movesthrough the borehole.

18. The method of claim 15 wherein said energy regions corresponding toinelastic gamma rays emitted by excited carbon, oxygen, silicon andcalcium are located approximately at the energy range of 3.17 MEV to4.65

MEV for carbon, 4.86 MEV to 6.34 MEV for oxygen, 1.65 MEV to L86 MEV forsilicon, and 2.5 MEV to 3.3 MEV for calcium.

19. The method of claim 15 wherein the step of combining the limefraction, the carbon/oxygen ratio and the porosity to compute the watersaturation S is performed by linearly interpolating between twopreselected carbon/oxygen ratio functions, f,() and f where f and f arefunctions of the porosity d: expressing the carbon/oxygen counts ratioin an oil sand and a water sand respectively, by an amount relateddirectly to the lime fraction.

1. A method of determining the water saturation of earth formationssurrounding a well bore comprising the steps of: determining thefractional content of sand and lime in the formation matrix; determiningthe porosity of the formation; determining the carbon/oxygen countsratio in the inelastic gamMa ray energy spectrum in regions thereofcorresponding to carbon and oxygen inelastic gamma ray and combining thefractional content of sand and lime, the porosity of the formation, andthe carbon/oxygen counts ratio in a predetermined manner to compute thewater saturation of the matrix.
 2. The method of claim 1 wherein thestep of determining the fractional content of sand and lime in theformation matrix is performed by repetitively irradiating the matrixwith high energy neutron pulses and measuring gamma radiation due to theinelastic scattering of neutrons by the formation matrix in pluralenergy ranges.
 3. The method of claim 2 wherein said plural energyranges are chosen to include gamma radiation resulting from theinelastic scattering of high energy neutrons by carbon, oxygen, siliconand calcium nuclei.
 4. The method of claim 3 wherein the energy rangesare chosen to be 3.17 MEV to 4.65 MEV for carbon, 4.86 MEV to 6.34 MEVfor oxygen, 1.65 MEV to 1.86 MEV for silicon and 2.5 MEV to 3.3 MEV forcalcium.
 5. The method of claim 1 wherein the step of determining thelime fraction LFX of an unknown formation X, is performed by measuringthe counts ratio RX of silicon/calcium counts resulting from gammaradiation due to inelastic fast neutron scattering in energy regions inthe gamma ray spectrum corresponding to the production of such gammaradiation by silicon and calcium nuclei in the unknown formation, andcombining this measurement according to the relationship LFX (RS -RX)/(RS - RL) where RS is the silicon/calcium counts ratio taken withthe same instrumentation in a known water sand formation and RL is thesilicon/calcium counts ratio taken with the same instrumentation in aknown water lime formation.
 6. The method of claim 5 and furtherincluding the step of logging the lime fraction LFX as a function ofborehole depth.
 7. The method of claim 1 wherein the step of determiningthe formation porosity is done by computing the porosity phi in apredetermined manner as a function of the count rates CR in energyregions of the gamma ray energy spectrum corresponding to known gammaray energies produced by the inelastic scattering of high energyneutrons by the nuclei of carbon, oxygen, silicon and calcium.
 8. Themethod of claim 7 wherein the porosity phi is computed from said countrate CR according to the relationship phi M e N CR where M and N arepredetermined calibration constants for a particular logging tool andborehole size and the constant e 2.718- - -, the Naperian logorithmbase.
 9. The method of claim 8 wherein CR is the count rate from thecarbon window only.
 10. The method of claim 8 and further including thestep of logging the porosity phi as a function of the borehole depth ofa well tool.
 11. The method of claim 1 wherein the step of combining thefractional content of sand and lime, the formation porosity, and thecarbon/oxygen counts ratio to compute the water saturation SW of theformation is performed by linearly interpolating by an amount (K.LFX),where K is a predetermined constant and LFX is the measured limefraction in the unknown formation, between two preselected carbon/oxygencounts ratio functions, f1( phi ) and f2( phi ), where f1( phi ) is afunction of porosity phi , expressing the carbon/oxygen counts ratio ina water sand and f2( phi ) is a function of porosity phi , expressingthe carbon/oxygen counts ratio in an oil sand.
 12. The method of claim11 wherein the function f1( phi ) and f2( phi ) are preselected to bepolynomial functions of porosity phi .
 13. The method of claim 11 andfurther Including the step of logging the water saturation SW as afunction of the borehole depth of a well tool.
 14. The method of claim 1and further including the step of logging the water saturation SW, theporosity phi , the carbon/oxygen counts ratio and the Si/Ca counts ratioall simultaneously as a function of the borehole depth of a well tool.15. A method of determining the characteristics of earth formationspenetrated by a well borehole comprising the steps of: passing a welltool having a pulsed neutron source and a gamma ray detector through awell bore; repetitively irradiating the earth formations surrounding thewell bore with high energy neutrons; measuring the count rates ofinelastic gamma rays in the gamma ray energy spectrum in energy regionsthereof corresponding to carbon, oxygen, silicon and calcium nucleiexcited by the high energy neutron pulses; determining the carbon/oxygencount rate ratio of the formation; determining, from the silicon andcalcium count rates, the lime fraction of the formation; determining,from at least one of the above count rates, the formation porosity; andcombining the lime fraction, the carbon/oxygen ratio and the porositydata to derive the water saturation of the formations penetrated by theborehole.
 16. The method of claim 15 wherein the neutron pulses have asshort a duration as practicable to eliminate interference in theinelastic gamma ray energy spectrum from the buildup of a thermalneutron population.
 17. The method of claim 15 and further including thestep of recording the water saturation SW, the porosity phi , the C/Oratio and the Si/Ca ratio as a function of borehole depth of said welltool as it moves through the borehole.
 18. The method of claim 15wherein said energy regions corresponding to inelastic gamma raysemitted by excited carbon, oxygen, silicon and calcium are locatedapproximately at the energy range of 3.17 MEV to 4.65 MEV for carbon,4.86 MEV to 6.34 MEV for oxygen, 1.65 MEV to 1.86 MEV for silicon, and2.5 MEV to 3.3 MEV for calcium.
 19. The method of claim 15 wherein thestep of combining the lime fraction, the carbon/oxygen ratio and theporosity to compute the water saturation SW is performed by linearlyinterpolating between two preselected carbon/oxygen ratio functions, f1(phi ) and f2( phi ) where f1 and f2 are functions of the porosity phiexpressing the carbon/oxygen counts ratio in an oil sand and a watersand respectively, by an amount related directly to the lime fraction.